Generally, as a way to convert three-phase alternating current (AC) power into direct current (DC) power, a three-phase full-wave rectification system has been used. However, in the three-phase full-wave rectification system, there are two occasions where an electrical potential of each phase comes to an intermediate potential of the other two phases in one cycle period with a section width of 60 degrees. Since the section is a non-conductive section where no current flows, input current results in a waveform containing harmonic current. Besides, the DC voltage thereof traces the maximum voltage of input line voltage, and thus it results in a pulsating waveform.
As a way to reduce non-conductive sections of input and to reduce harmonic current, a passive method is used in which reactors of large capacity are connected to the input to store power and then to release the power during non-conductive sections to reduce the non-conductive sections. As a way to minimize non-conductive sections of input, an active method is used in which a PFC circuit or a PWM rectifier circuit is configured with semiconductors to compensate the non-conductive sections of the input with reactors of small capacity. As a way to remove DC voltage ripple, in both the passive and active methods, a capacitor of large capacitance is connected on the DC side.
Of the conventional methods, a passive system requires storage of power. Because reactors and capacitors of large capacity are required to store power in this system, it is costly and the circuit size becomes larger.
Meanwhile, in an active system, generally, DC voltage is detected to control input current, and thus a stable DC voltage is required. Because it is necessary to select a DC intermediate capacitor as a power storage device in consideration of fluctuation of DC voltage induced by fluctuation of load or input voltage, reduction in the capacitance thereof is limited. Further, the control is complicated, and a load of high withstand voltage is used since there is provided only a voltage step-up function, resulting in a high cost.
As a current type three-phase step-down rectifier used in an active system other than the above, for example, Patent Documents 1 and 2 are known. In Patent Document 1, a current type step-down converter is configured with self-extinguishing switching elements, and a section in which DC voltage is set to 0 by switching off all pulses or by a short-circuit pulse is inserted to the output of a rectifier to smooth out DC voltage and to reduce harmonic current. In Patent Document 2, downsizing of a DC reactor in this system is proposed. However, in the systems, DC voltage is not intended to be constant and components of power supply frequency cannot completely be eliminated, and thus the capacity of the reactor and the capacitor to absorb the power supply frequency components need to be at or above a certain level corresponding to a power supply frequency (for example, 50 Hz).